CST121 Midterm Exam

NOTES:
• This "midterm" exam is to be done in class.
• The "takehome" exam is exactly the same, so you will have a second chance to do a better job on it, later!
• For both exams, upload the files to your folder in suffolk.li

• For the in-class exam, name your files:  midterm.asm   &   midterm.txt
• For the in-class exam, name your files:   takehome.asm   &   takehome.txt
• Upload the midterm files today; upload the takehome files by 12 noon, March 31st.

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  1. Write a complete program to perform the following tasks.

  Assemble, execute, and test your program, then upload your source code into your folder,
  using the file name midterm.asm (or takehome.asm for the improved version).
  TASKS:

  A. Input 4 numbers.

  Prompt the user to enter four (unsigned) numbers. (A single prompt message is sufficient.)
  and store them in memory locations named P, Q, R, and S.

  B. Compute the sum:

  Compute the sum total of these four numbers, and output a message giving the answer,
  including some text that says what it is; for example:    The total is 1234

  C. Compute the average:

  Compute the mean average, by dividing the total by four. Output a message giving the answer.
  Do NOT use an 8086 "divide" instruction to divide by 4 -- instead, use a "shift" instruction.

  D. Exclusive or of the first two numbers:   P = P xor Q

  Perform an "exclusive or" operation between the first two numbers, and store the result in P (destroying the first input value).

  E. Exclusive or the reult with second number:

  Perform an "exclusive or" operation between "Q" and (the new value in) "P",
  and store the result into Q (destroying the second input value).

  F. Now, output the new values of P and Q (with text identifying which is which). For example:

        The new value of P is 5678

        The new value of Q is 1234

  G. Masking out:

  Mask out the first 3 and last 3 bits of the value in "R" (keeping the ten bits in the middle)
  and output the value the result in hexidecimal
  (but do NOT store the result into R.)

  H. Forcing a bit on.

  Force the lowest bit of "S" to be a one (regardless of whether it WAS 1 or 0).
  QUESTION:   What does this do to the value that was previously in S?
  Output a message that displays the new value of S, and answers the above question.

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  2. Complete the table that appears below.

  Copy your completed table into a file named midterm.txt and upload it to your folder.
  For the takehome exam, upload a file named takehome.txt and upload it to your folder.

  Offset Machine-code Assembly-code: .DATA ... 0456 6789 Z DW 6789h 0458 5678 Y DW 5678h 045A 3456 X DW 3456h 045C 2345 W DW 2345h 045E 57 68 61 74 65 76 65 72 OTHER DB 'Whatever' 0466 58 F DB 'X' 0467 4C 61 73 74 6E 61 6D LAST DB 'Lastname' ;;;; YOUR own Initial & Lastname. ;;;; (NOT 'X' or 'Lastname' !) ... .CODE 0000 MID PROC 0000 B8 ---- R mov ax, @data 0003 8E D8 mov ds, ax 0005 8A 1E 045E R mov BL, [OTHER] 0009 B7 FB mov BH, 251 000B 03 1E 045C R add BX, [W] 000F 77 04 ja M1 0011 88 1E 045E R mov [OTHER], BL 0015 M1: 0015 8A 0E 0466 R mov CL, [F] ; YOURS (not 'X') 0019 B5 7B mov CH, 123 001B 03 0E 0456 R add CX, [Z] 001F 77 04 ja M2 0021 89 0E 0458 R mov [Y], CX 0025 M2: 0025 BA 04D2 mov DX, 1234 ; (Decimal) 0028 B9 0005 mov CX, 5 002B D3 C2 rol DX, CL 002D 89 16 045A R mov [X], DX ...

  
  
  

  NOTE: You may ignore the Fetch-Execute Cycle, which performs operations such as the following: Move 16-bits of Address from IP to Memory Move 8-bit Instr. bytes from Memory to IR etc., etc.

  Below is a description of the "steps" that are performed by the first few instructions in the code above.

  As shown in the table below, each instruction is "executed" by performing one or more "steps" -- where each "step" consists of MOVING some BITS from one place to another, as follows:
  

  • Move some NUMBER of bits:   8 or 16 or 32
  • OF either Address or Data:   A or D
  • FROM some place, such as:   Memory [M] , ALU output ALU,    the instruction pointer IP, instruction register IR, or another register AL, AH, AX, etc.
  • TO some place, such as:   Memory [M] , ALU output ALU,    the instruction register IR, or another register AL, AH, AX, etc.

  In other words:   each instruction is executed by a series of "steps" that move 8 or 16 or 32 bits, of either Address or Data -- with the value given in hexadecimal -- from one place to another.

  Complete the table below, to describe each step.
  For your convenience, there is a copy of this table in the text file at midterm.txt .

  
  				#bits	of	Value	FROM:		TO:
  				8/16/32	(A/D)	(hex)	 M, ALU, IR, AX, etc...
  
  	mov	BL, [A]
  			  Move	16-bit	Address 045E	from IR    	to Memory
  			  Move	8-bit	Data	57  	from Memory	to Register BL
  	mov	BH, 241
  			  Move	8-bit	Data	FB  	from IR 	to Register BH
  
  	Continuing this table, with some unnecessary words omitted:
  
  				#bits	of	Hex	FROM:	TO:
  	add	BX, [W]
  				16	A	045C	IR    	M
  				16	D	2345	M	ALU
  				16	D	FB57	BX    	ALU
  				16	D	1EAB	ALU 	M
  
  
  Now, complete the table for the remaining instructions.
  
  All values should be shown in hexadecimal (without a suffix "h").
  Be sure to use YOUR first and last name.
  (Don't use "Lastname" or "X" -- unless that's really your initial!)
  
  When you are done, upload a text file "midterm.txt" (or "takehome.txt"),
  containing the instructions and your answers for each of the "steps".
  The file midterm.txt contains the text below, or you copy and paste (or create your own).
  

  #bits of Hex FROM: TO: Offset Assembly-code: 0005 mov BL, [OTHER] 16 Address 045E IR Memory 8 Data 57 M BL register 0009 mov BH, 251 8 D FB M BH 000B add BX, [W] 16 A 045C IR M 16 D 2345 M ALU 16 D FB57 BX ALU 16 D 1E9C ALU M 000F ja M1 _ _ _ _ _   _   _   _   0011 mov [OTHER], BL _ _ _ _ _   _   _   _   0015 M1: 0015 mov CL, [F] _ _ _ _ _   _   _   _   0019 mov CH, 123 _ _ _ _ _   _   _   _   001B add CX, [Z] _ _ _ _ _   _   _   _   001F ja M2 _ _ _ _ _   _   _   _   0021 mov [Y], CX _ _ _ _ _   _   _   _   0025 M2: 0025 mov DX, 1234 _ _ _ _ _   _   _   _   0028 mov CX, 5 _ _ _ _ _   _   _   _   002B rol DX, CL _ _ _ _ _   _   _   _   002D mov [X], DX _ _ _ _ _   _   _   _